I have been to Florence twice now and both times I stayed at the Suite Corso 12 with the breathtaking view of the Duomo. Samuel and Paolo are great guys and help with anything - we, unfortunately, had some very noisy neighbours who kept us awake into the early hours and Firenze Rentals did their best to resolve the situation, even so late at night (1am!).
The apartment is well equipped with a full kitchen, bathroom etc. (it now has an elevator for luggage etc., very handy!) and everything is in walking distance. If you go to Florence, you really should stay here. Especially when you share the costs between 3 or 4 people, it's a bargain.